For example, a copper wire material formed of low-oxygen copper such as tough pitch copper containing approximately 0.02 mass % to 0.05 mass % of oxygen or oxygen-free copper having an oxygen content of 10 ppm by mass or less, may be provided as a copper wire material used in a wire of an electrical wire, a lead wire, a magnet wire of a motor, or the like. Here, in a case of using a copper wire material for welding, for example, hydrogen embrittlement may occur when the oxygen content is great. Therefore, a copper wire material formed of low-oxygen copper such as oxygen-free copper is used.
Conventionally, the copper wire material described above is manufactured by dip forming or extrusion. In the dip forming, molten copper is continuously solidified on the outer periphery of a copper seed rod to obtain a rod-like copper material and this is rolled to obtain a copper wire material. In the extrusion, a billet of copper is subjected to extrusion and rolled or the like to obtain a copper wire material. However, in such manufacturing methods, productivity is poor and the production cost is high.
As a method for producing a copper wire material with a low production cost, a method performed by continuous casting rolling using a belt-caster type continuous casting apparatus (belt-wheel type continuous casting apparatus) and a continuous rolling apparatus may be used, as disclosed in PTL 1, for example. In this continuous casting rolling method, which is a method of cooling and solidifying molten copper melted in a large-sized melting furnace such as a shaft furnace to obtain a copper ingot and continuously withdrawing and rolling this copper ingot, mass production can be realized with a large-scale plant.
However, in a case where low-oxygen copper such as oxygen-free copper is manufactured as an ingot, a hydrogen concentration in molten copper increases and air bubbles of water vapor are generated. In addition, since a mold is rotationally moved in a belt-caster type continuous casting apparatus (belt-wheel type continuous casting apparatus), the generated air bubbles are difficult to remove from the surface of the molten copper and remains in the copper ingot, so that void defects are generated.
It is considered that such void defects remaining in the copper ingot are a main cause of surface defects of a copper wire material. The surface defects of the copper wire material causes surface defects in a drawn wire material, even in a case where a drawn wire material is obtained by executing a drawing process. In a case where this drawn wire material is used as a conductor of a magnet wire and an enamel coat (insulating film) is applied to the surface of the drawn wire material, water or oil remaining in a surface defect of the drawn wire material is retained in the enamel coat, and a defect called a “blister” of blistering of the enamel coat due to generation of air bubbles in the enamel coat, when heat is applied after drying the enamel coat, may occur.
In order to prevent generation of void defects in a copper ingot and surface defects in a copper wire material, PTL 2, for example, discloses a copper ingot which is manufactured by adding a phosphorous compound to molten copper so that the phosphorous content of an ingot becomes 1 ppm to 10 ppm and adjusting a temperature of the molten copper in a tundish to 1085° C. to 1100° C., and a copper wire material.
However, in the copper wire material disclosed in PTL 2, since the amount of phosphorus is as low as 1 ppm to 10 ppm, it is difficult to fix oxygen in the molten copper as the phosphorous compound and it is difficult to sufficiently prevent generation of air bubbles of water vapor. Accordingly, it is difficult to prevent generation of void defects in the copper ingot and to sufficiently reduce surface defects generated in a copper wire material.
Meanwhile, PTL 3 does not disclose a casting using a belt-caster type continuous casting apparatus (belt-wheel type continuous casting apparatus), but proposes a technology of promoting a reaction between oxygen and carbon to improve deoxidation efficiency, by bubbling an inert gas into a molten metal launder in which a solid reducing agent such as charcoal powder is disposed on a surface of molten copper in a method for producing P-containing low-oxygen copper in which the oxygen content is 10 ppm or less and to which 10 ppm to 140 ppm of phosphorus is added. In PTL 3, gas components in the molten copper are determined by a partial pressure balancing method, but PTL 3 does not disclose gas components in the copper ingot.